Conventional photographic materials based on silver halide are used for a large variety of applications. As is generally known silver halide materials have the advantage of high potential intrinsic sensitivity and excellent image quality. On the other hand they show the drawback of requiring several wet processing steps employing chemical ingredients which are suspect from an ecological point of view.
In the past several proposals have been made for obtaining an imaging element that can be developed using only dry development steps without the need of processing liquids as it is the case with silver halide photographic materials.
A dry imaging system known since quite a while is 3M's Dry Silver technology. Another type is based on photopolymerisation.
As a further alternative for silver halide chemistry dry imaging elements are known that can be image-wise exposed using an image-wise distribution of heat. When this heat pattern is applied directly by means of a thermal head such elements are called thermographic materials. When the heat pattern is applied by the transformation of intense laser light into heat these elements are called heat mode materials or thermal imaging media.
A particular type of heat mode materials is based on the chemical reduction of organic silver salts. Another category is based on change of adhesion.
In still another particular type of heat mode recording information is recorded by creating differences in reflection and/or in transmission on the recording layer. The recording layer has high optical density and absorbs radiation beams which impinge thereon. The conversion of radiation into heat brings about a local temperature rise, causing a thermal change such as evaporation or ablation to take place in the recording layer. As a result, the irradiated parts of the recording layer are totally or partially removed, and a difference in optical density is formed between the irradiated parts and the unirradiated parts (cf. U.S. Pat. Nos. 4,216,501, 4,233,626, 4,188,214 and 4,291,119 and British Pat. No. 2,026,346)
The recording layer of such heat mode recording materials is usually made of metals, dyes, or polymers. Recording materials like this are described in `Electron, Ion and Laser Beam Technology", by M. L. Levene et al.; The Proceedings of the Eleventh Symposium (1969); "Electronics" (Mar. 18, 1968), P. 50; "The Bell System Technical Journal", by D. Maydan, Vol. 50 (1971), P. 1761; and "Science", by C. O. Carlson, Vol. 154 (1966), P. 1550.
Recording on such thermal recording materials is usually accomplished by converting the information to be recorded into electrical time series signals and scanning the recording material with a laser beam which is modulated in accordance with the signals. This method is advantageous in that recording images can be obtained on real time (i.e. instantaneously). Recording materials of this type are called "Direct Read After Write" (DRAW) materials. DRAW recording materials can be used as a medium for recording an imagewise modulated laser beam to produce a human readable or machine readable record.
According to EP 0 384 041 a process is provided for the production of a heat mode recording material having DRAW possibilities wherein a web support is provided with a heat mode recording thin metal layer, preferably a bismuth layer, characterized in that in the same vacuum environment a protective organic resin layer in web form is laminated to said supported recording layer by means of an adhesive layer.
A commercially available material manufactured according to the principles of cited EP 0 384 041 is MASTERTOOL MT8, registered trade name, marketed by Agfa-Gevaert N.V. It is mostly used after recording as master in the manufacturing of microelectronic circuits and printed circuit boards. We refer to the description in Circuit World, Vol. 22, No. 3, April 1996.
In EP 0 687 569 a further alternative to the teachings of EP 0 384 041 is disclosed. In this embodiment, the adhesive layer plus organic resin web is replaced by a single adhesive layer, or by a multiple layer assemblage, applied by lamination in the vacuum chamber, whereby said single adhesive layer or the outermost layer of said assemblage is hardened by heat and/or by ultra-violet radiation or electron beam radiation.
A drawback of the method of preparation of a thin bismuth recording layer by vacuum deposition is the fact that this is a complicated, cumbersome and expensive process.
Therefore, in European patent application appl. No. 9720182 the vacuum deposition is replaced by coating from an aqueous medium. According to this disclosure a thin metal layer is formed by the following steps:
(1) preparing an aqueous medium containing ions of a metal, PA1 (2) reducing said metal ions by a reducing agent thus forming metal particles, PA1 (3) coating said aqueous medium containing said metal particles on a transparent support. PA1 (A) providing a material having following layer arrangement, in order: PA1 (B) radiation curing said coated single top layer (3). PA1 (i) preparing an aqueous solution containing the corresponding metal ions, PA1 (ii) reducing said metal ions by a suitable reducing agent thus forming metal particles.
In the preferred embodiment of the cited invention the metal is bismuth.
In this embodiment the laminated protective element disclosed in EP 0 687 569 can simply be replaced by a coated double layer pack, a first weak layer having a good adherance to the metal layer, and a top protective hard layer. Examples of such double layer packs are disclosed in example 4 of the cited application. A drawback from this procedure is the need for a double layer which implies a more complicated coating procedure.
The present invention extends the teachings on heat mode materials based on a thin metal recording layer coated from an aqueous medium.